Glasses for the correction of chromatic and thermal optical aberations for lenses transmitting in the near, mid, and far-infrared spectrums

ABSTRACT

The invention relates to chalcogenide glass compositions for use in a lens system to balance thermal effects and chromatic effects and thereby provide an achromatic and athermal optical element that efficiently maintains achromatic performance across a broad temperature range. The glass composition is based on sulfur compounded with germanium, arsenic and/or gallium, and may further comprise halides of, for example, silver, zinc, or alkali metals. Alternatively, is based on selenium compounded with gallium, and preferably germanium, and contains chlorides and/or bromides of, for example, zinc, lead or alkali metals.

SUMMARY OF THE INVENTION

The invention relates to glass compositions that can be used formanufacturing optical lens that correct for optical aberrations,particularly chromatic aberrations and aberrations due to thermaleffects, of lens that transmit light in the near-, mid- and/or far-rangeinfrared spectrum, and preferably also within at least a portion of thevisible spectrum.

Infrared lens transmit light in near-infrared range (e.g., 700 nm to 1.8μm), the mid-infrared range (e.g., 3.0-5.0 μm) and/or the far-infraredrange (e.g., 8.0-13.0 μm). Often IR lenses are characterized astransmitting light in the SWIR, MWIR, or LWIR regions, i.e., theshort-wave (SWIR) region (wavelengths of 1-3 μm), mid-wave (MWIR) region(wavelengths of 3-5 μm), and the long-wave (LWIR) region (wavelengths of8-12 μm). Infrared lens are used in a wide variety of applicationsincluding low-light level (night vision) imagers such as night visiongoggles, thermal imagers, and systems capable of seeing throughobscurants such as fog, smoke and dust.

Night vision devices such as night vision goggles generally rely onlow-level reflected light in the visible and near-infrared range. Thesedevices utilize image enhancers that collect the visible and infraredlight passing through the lens and amplify the light to produce avisible image. In general, night vision goggles comprise an infraredobjective lens which transmits light in the visible and near-infraredrange, an image enhancer or intensifier that amplifies the photons andconverts them to electrons, and a phosphor or fluorescent display thatreceives the electrons and produces an amplified image. See, forexample, Filipovich (U.S. Pat. No. 4,653,879).

Thermal imagers utilize emitted, rather than reflected, infrared light,specifically emitted thermal energy. Therefore, thermal imagersgenerally operate in the mid-infrared s and/or the far-infrared ranges.Humans, animals, and operating machines, for example, produce their ownheat which is emitted as infrared radiation. Other objects rocks andbuildings absorb heat from the sun, for example, and then radiate thatheat as infrared light. Thus, thermal imagers have many civilian andmilitary applications for purposes of surveillance, security and safety,such as imaging people and vehicles, determining hot spots, andmonitoring industrial machinery and processing plants.

In general, an infrared or thermal imaging system comprises opticsincluding an IR lens for collecting and focusing transmitted infraredlight and a plurality of thermal sensors for detecting the infraredlight and converting it into electrical signals, and a signal-processingunit for converting the electrical signals into a visual image. See, forexample, Izumi (U.S. Pat. No. 7,835,071).

Optical lens including infrared lens are susceptible to several opticalaberrations. For example, most imaging systems need to bring light ofmany wavelengths to a focus at the same distance from the lens. However,the refractive index of all known materials varies as a function of thewavelength. This variation in refractive index, known as dispersion,produces an aberration known as chromatic aberration, sometimes referredto as “color fringing.”

There are two types of chromatic aberration. Longitudinal chromaticaberration or axial chromatic aberration results when the differentwavelengths transmitted by the lens have different focal lengths, sincethe focal length of a lens varies as a function of its refractive index.As a result, the wavelengths do not focus on the same focal plane. So,for example, the focal distance of blue light will be shorter than thefocal distance for red light.

Lateral chromatic aberration occurs when the different wavelengths aremagnified differently by the lens. As a result, the wavelengths willfocus at different positions along the same focal plane.

One approach to overcoming chromatic aberration is to use multiplelenses to counter-act the influence of refractive index dispersion onthe image. An achromat lens or achromatic doublet is made by combiningtwo different lens materials that have different dispersion properties.The achromat lens functions to bring two different wavelengths both intofocus on the same focal plane, thereby reducing chromatic aberration.

Apochromatic lenses involve multiple materials and are designed to bringthree or more wavelengths into focus in the same plane. Such lensesprovide better correction of chromatic aberration and also alleviatespherical aberration (i.e., an aberration that occurs when light passingthrough a lens is refracted more at the lens's edge than at its center).Thus, the use of such doublet or triplet (or greater) lenses mayalleviate the phenomenon of chromatic aberration and thereby improvecolor rendering of an optical system.

For lenses that transmit primarily in visible spectrum, the use ofdoublet or triplet lenses is common practice. One can select two, or inmany cases three or even more, materials from a wide range of availableglass types, and tune the lens design to the desired opticalperformance. However, the design of such multiple lens arrangements ismore difficult for infrared lenses. The number of optical materials thatare transparent in the mid- and far-infrared range is very limited. Suchdesign is even more complicated when transparency in the visible (400 nmto 800 nm) or near-infrared (700 nm to 1.8 μm) is requiredsimultaneously with and mid-infrared (3.0-5.0 μm) and/or far-infrared(8.0-13.0 μm) transmission.

In addition to dispersion, most infrared-transparent materials sufferfrom a large temperature dependence of the refractive index and fromlarge coefficients of thermal expansion. Both of these factors inducechanges in the focal length of a lens as the temperature changes,leading to thermal defocusing. Thus, in addition to addressing theproblem of chromatic aberration by providing achromatic infrared lenssystems, it is also desirable to provide athermal infrared lens systemsin which the optical performance is stabilized with respect tovariations in temperature.

For a description of prior art attempt to achieve athermalization of IRlens systems, see, for example, Jamieson, T. H., Athermalization ofOptical Instruments from the Optomechanical Viewpoint, Proc. SPIE, CR43,131 (1992).

In addition, Arriola (U.S. Pat. No. 5,737,120) discloses an achromaticand athermal two element objective lens that transmits in the long waveinfrared (LWIR) spectral region (8-12 μm). One lens element of theobjective lens is made of zinc selenide (ZnSe) and has a positiveoptical power. The other lens element is made of germanium (Ge) and hasa negative optical power. The positive lens element has a lowerthermo-optic coefficient (lower dn/dT) than the negative lens. Thisdifference in thermo-optic coefficient provides for athermalization ofthe lens system, but not color correction. To provide color correction,Arriola attaches a diffractive optical surface on one surface of eitherlens element.

From an optical perspective, the halides (F, Cl, Br and I) of silver(Ag), thallium (Tl), and the alkali metals (Na, K, Rb and Cs) areattractive materials for attempting to fulfill the requirements of anachromatic and athermal compound IR lens. However, these materialssuffer from extremely low mechanical durability, high toxicity, and, inthe case of the alkali metals, extreme sensitivity to moisture.Therefore, the use of these materials is commonly seen as impractical.

Other polycrystalline materials that could possibly satisfy the desiredcriteria include polycrystalline compounds of alkaline earth elements(Ca, Sr, Ba) with fluorine and compounds of zinc (Zn) with group IV“chalcogenide” elements (S, Se). These materials are known to havesufficient chemical and mechanical durability. However, the combinationof their particular refractive indices and dispersions are not suitablefor practical achromatic optics. Moreover, the fluorides tend to lacksufficient transmission at wavelengths beyond 10 μm. Intrinsicsemiconductor materials composed of Group IV elements (Si and Ge) orcompounds of group III and group V elements such as GaAs and InSb do notsimultaneously offer sufficient mid/far-IR and visible/near-IRtransparency.

Since the chemical composition of crystalline compounds is fixed, it isnot possible to tune their properties to allow achromatic performance ina two-element lens system through varying the composition. On the otherhand, glasses which offer both infrared and visible transparency might,by compositional tailoring, be used to balance the chromatic effects ofother glasses or crystalline materials in a compound IR lens. However,to date no glasses are available that have properties tuned to satisfythe requirements of achromatic and athermal optical element forbroadband optics. It is possible to achieve achromatic and athermalperformance using a large number of crystalline compounds, often usinggreater than 5 individual optical elements. But, such designs are costlydue to added mechanical complexity and the need for many speciallydesigned anti-reflection coatings, or such designs have poor performancedue to large reflection losses at the various interfaces. Additionally,most of the available crystalline materials, such as KBr or KRS5(thallium bromo-iodide; TlBr-TlI) suffer poor mechanical and chemicalstability and may be highly toxic.

Therefore, an aspect of the invention is to provide glass compositions,in particular chalcogenide glass compositions, for use in a lens systemthat simultaneously balances both the thermal effects and chromaticeffects of multiple lenses within a compound optical element to achievean infrared optical system that will efficiently maintain achromaticperformance across a broad temperature range, and preferably is suitablefor use in broadband optics.

Upon further study of the specification and appended claims, furtheraspects and advantages of this invention will become apparent to thoseskilled in the art.

According to one aspect of the invention, there is provided a glasscomposition based on sulfur compounded with germanium, arsenic and/orgallium that may further comprise halides of silver, copper (Cu⁺¹),cadmium, zinc, lead (Pb⁺²), alkali metals, alkaline earth metals, orrare earth metals, wherein the glass composition transmits near-, mid-,and/or far-infrared light. The glass system based on sulfur compoundedwith germanium, arsenic and/or gallium provides compositions withrelatively low refractive indices. Moreover, these glass compositionsexhibit relatively low refractive index dispersion in the mid-infraredrange, although the refractive index dispersion in the near- andfar-infrared can be high. The optional halides provide the ability tonot only enhance infrared transparency of the glass, but also aid incontrolling refractive index dispersion and thermal expansion.

According to a further aspect of the invention, there is provided achalcogenide glass composition based on sulfur compounded withgermanium, arsenic and/or gallium, comprising (based on mol %):

Component Mole % S 58.00-90.00   Ga  0-25.00 As 0-40.0 Ge  0-35.00 R¹0-7.25 (added in the form of R¹Hal) R² 0-13.5 (added in the form ofR²Hal) M¹ 0-5   (added in the form of M¹Hal₂) M² 0-7.25 (added in theform of M²Hal₂) Ln 0-4   (added in the form of LnHal₃) Sum of Ga, As,and Ge 10.00-42.00   Sum of R¹, R², M¹, M², and Ln  0-16.00 Sum of Hal 0-16.00

wherein

-   -   Hal=fluoride, chloride, bromide, and/or iodide,    -   R¹=Li, Na, K, Rb, and/or Cs,    -   R²=Ag and/or Cu,    -   M¹=Mg, Ca, Sr, and/or Ba,    -   M²=Zn, Cd, Hg, and/or Pb,    -   Ln=La, Ce, Pr, Nd, Pm, Sm Eu, Gd, Tb, Dy, Ho, Er, Tm, Ty, Lu, Y,        and Sc; and    -   wherein a portion of the gallium can be replaced by indium, and        a portion of the arsenic can be replaced by antimony.

The glass system based on sulfur compounded with germanium, arsenicand/or gallium, at a thickness of 10 mm, preferably transmits at least75% of incident light at wavelengths from 500 nm to 11000 nm, especiallyat least 70% of incident at wavelengths from 650 nm to 12000 nm, andparticularly at least 70% of incident at wavelengths from 500 nm 14000nm.

The glass system based on sulfur compounded with germanium, arsenicand/or gallium also preferably exhibits an extinction coefficient of<0.1 cm⁻¹ at wavelengths from 500 nm to 11000 nm, especially atwavelengths from 650 nm to 12000 nm, and particularly at wavelengthsfrom 500 nm 14000 nm

According to another aspect of the invention, there is provided a glasscomposition based on selenium compounded with gallium, and containing alarge of chlorides and/or bromides of silver, copper (Cu⁺¹), cadmium,zinc, mercury, lead (Pb⁺²), alkali metals, alkaline earth metals, orrare earth metals, wherein the glass composition transmits near-, mid-,and/or far-infrared light. These glasses offer enhanced infraredtransmission, and lower far- infrared dispersion, but requiresignificantly higher additions of halides to achieve high visibletransmission.

According to a further aspect of the invention, there is provided achalcogenide glass composition based on selenium compounded with galliumand optionally germanium, comprising (based on mol %):

Component Mole % Se 30.00-68.00    Ga 5.00-30.00   Ge 0-25.00 R¹ 0-25.00(added in the form of R¹Hal¹) R² 0-25.00 (added in the form of R²Hal¹)M¹ 0-12.50 (added in the form of M¹Hal¹ ₂) M² 0-20.00 (added in the formof M²Hal¹ ₂) Ln 0-8.00  (added in the form of LnHal¹ ₃) Sum of Se, Ga,and Ge 50.00-93.33    Sum of R¹, R², M¹, M², and 1.67-25.00   Ln Sum ofHal¹ 5.00-25.00  

wherein

-   -   Hal¹=chloride and/or bromide,    -   R¹=Li, Na, K, Rb, and/or Cs,    -   R²=Ag and/or Cu,    -   M¹=Mg, Ca, Sr, and/or Ba,    -   M²=Zn, Cd, Hg, and/or Pb,    -   Ln=La, Ce, Pr, Nd, Pm, Sm Eu, Gd, Tb, Dy, Ho, Er, Tm, Ty, Lu, Y,        and Sc; and    -   wherein a portion of the gallium can be replaced by indium.

The glass system based on selenium compounded with gallium, andcontaining a chlorides and/or bromides, at a thickness of 10mm,preferably transmits at least 75% of incident light at wavelengths from500 nm to 11000 nm, especially at least 70% of incident at wavelengthsfrom 650 nm to 12000 nm, and particularly at least 70% of incident atwavelengths from 500 nm 14000 nm.

The glass system based on selenium compounded with gallium, andcontaining a chlorides and/or bromides also preferably exhibits anextinction coefficient of <0.1 cm⁻¹ at wavelengths from 500 nm to 11000nm, especially at wavelengths from 650 nm to 12000 nm, and particularlyat wavelengths from 500 nm 14000 nm.

For both the sulfur based compositions and the selenium basedcompositions, the properties of most interest, in addition to goodchemical and mechanical durability and desired light transmission, areindex dispersion, coefficient of thermal expansion, and thermaldependency of refractive index.

The index dispersion is preferably as low as possible. The amount ofindex dispersion is measured as the Abbe number in the visible, V_(d),which is calculated as V_(d)=(n_(d)−1)/(n_(F) −n _(c)) where n_(d),n_(F) and n_(c) are the refractive indices of the material at the dline, F line, and C line (F line: 486.13 nm, d line: 587.56 nm, C line:656.27 nm). Abbe number in the mid-IR range (3-5 μm) is generallycalculated using the index at 3000, 4000, and 5000 nm while the Abbenumber in the long-wave range (8-12 μm) may be calculated using theindex at 8000, 10,000 and 12,000 nm

In general, the higher the Abbe No. the lower index dispersion. Theglass compositions according to the invention preferably exhibit an AbbeNo. in the visible range of at least 15, for example, 20-30, especiallygreater than 25. In the mid-infrared range the glasses preferablyexhibit an Abbe No. of at least 100, for example, 100-300, especially atleast 180, particularly greater than 200. In the far-infrared range theglasses preferably exhibit an Abbe No. of at least 60, for example,60-185, especially at least 100, particularly greater than 120.

Similarly, the coefficient of thermal expansion, a, is preferred to beas low as possible for the glass compositions according to theinvention. Thus, the glasses according to the invention preferably havea coefficient of thermal expansion that is less than 50×10⁻⁶/K orexample, 15×10⁻⁶/K-25×10⁻⁶/K.

The thermal dependency of the refractive index, measured as dn/dT (thetemperature coefficient of the refractive index), is also preferablylow. Thus, the glasses according to the invention preferably have adn/dT value of less than 30×10⁻⁶/K, for example, 5×10⁻⁶/K-30×10⁻⁶/K.

According to an aspect of the invention, there is provided a glasscomposition based on sulfur compounded with germanium, arsenic and/orgallium, the glass composition comprising (based on mol %):

Component Mole % S 58.00-90.00    Ga 0-25.00 As 0-40.0  Ge 0-35.00 R¹0-7.25  (added in the form of R¹Hal) R² 0-13.5  (added in the form ofR²Hal) M¹ 0-5    (added in the form of M¹Hal₂) M² 0-7.25  (added in theform of M²Hal₂) Ln 0-4.00  (added in the form of LnHal₃) Sum of Ga, As,and Ge 10.00-42.00    Sum of R¹, R², M¹, M², and Ln 0-16.00 Sum of Hal0-16.00

wherein

-   -   Hal=fluoride, chloride, bromide, and/or iodide,    -   R¹=Li, Na, K, Rb, and/or Cs,    -   R²=Ag and/or Cu,    -   M¹=Mg, Ca, Sr, and/or Ba,    -   M²=Zn, Cd, Hg, and/or Pb,    -   Ln=La, Ce, Pr, Nd, Pm, Sm Eu, Gd, Tb, Dy, Ho, Er, Tm, Ty, Lu, Y,        and Sc.

According to a further aspect, the invention includes a glasscomposition, based on sulfur compounded with germanium, arsenic and/orgallium, comprising (based on mol %):

Component Mole % S 65.00-75.00 Ga    0-10.00 As    0-35.00 Ge 3.00-30.00 R¹ 0-5 (added in the form of R¹Hal) R²  0-10 (added in theform of R²Hal) M¹ 0-3 (added in the form of M¹Hal₂) M² 0-5 (added in theform of M²Hal₂) Ln 0-3 (added in the form of LnHal₃) Sum of Ga, As, andGe 30.00-40.00 Sum of R¹, R², M¹, M²,  0-10 and Ln Sum of Hal  0-10

According to another aspect of the invention, there is provided a glasscomposition based on selenium compounded with gallium, the glasscomposition comprising (based on mol %):

Component Mole % Se 30.00-68.00    Ga 5.00-30.00   Ge 0-25.00 R¹ 0-25.00(added in the form of R¹Hal¹) R² 0-25.00 (added in the form of R²Hal¹)M¹ 0-12.50 (added in the form of M¹Hal¹ ₂) M² 0-20.00 (added in the formof M²Hal¹ ₂) Ln 0-8    (added in the form of LnHal¹ ₃) Sum of Se, Ga,and Ge 50.00-93.33    Sum of R¹, R², M¹, M², and 1.67-25.00   Ln Sum ofHal¹ 5.00-25.00  

wherein

-   -   Hal¹=chloride and/or bromide,    -   R¹=Li, Na, K, Rb, and/or Cs,    -   R²=Ag and/or Cu,    -   M¹=Mg, Ca, Sr, and/or Ba,    -   M²=Zn, Cd, Hg, and/or Pb, and

Ln=La, Ce, Pr, Nd, Pm, Sm Eu, Gd, Tb, Dy, Ho, Er, Tm, Ty, Lu, Y, and Sc.

According to a further aspect, there is provided a glass composition,based on selenium compounded with gallium, comprising (based on mol %):

Component Mole % Se 35.00-65.00 Ga  7.00-22.00 Ge 18.00-23.00 R¹  0-20(added in the form of R¹Hal¹) R²    0-20.00 (added in the form ofR²Hal¹) M¹  0-10 (added in the form of M¹Hal¹ ₂) M²    0-15.00 (added inthe form of M²Hal¹ ₂) Ln 0-5 (added in the form of LnHal¹ ₃) Sum of Se,Ga, and Ge 55.00-85.00 Sum of R¹, R², M¹, M², and  1.67-22.00 Ln Sum ofHal¹  7.5-22.00

With regards to the sulfur based compositions, the amount of sulfur is58.00-90.00 mol %, preferably 58.00-75.00 mol %, based on the totalmoles (e.g., based on total moles of S, Ga, As, Ge, R¹, R², M¹, M², Lnand Hal when neither In nor Sb are present). According to anotheraspect, the sulfur based glass compositions according to the inventioncontain 65.00-75.00 mol % of sulfur, for example, 60.00-65.00 mol % ofsulfur, or 70.00-75.00 mol % sulfur, or 65.00-70.00 mol % sulfur.

Also, in the sulfur based compositions the amount of gallium is 0-25.00mol %, based on the total moles (e.g., based on total moles of S, Ga,As, Ge, R¹, R², M¹, M², Ln and Hal). According to another aspect, thesulfur based glass compositions according to the invention contains0-20.00 mol % Ga, for example, 0-10.00 mol % Ga, 5.00-15.00 mol % Ga, or5.00-10.00 mol % Ga, or 6 mol %, 7 mol %, 8 mol %, or 9 mol %.

According to another aspect, in the sulfur based glass compositionsaccording to the invention a portion of the gallium can be replaced byindium, particularly in situations were a lower amount of visibletransmission is acceptable. The presence of In tends to reduce visibletransmission. However, the combined total amount of gallium and indiumis still preferably 0-25 mol %, based on the total moles (e.g., based ontotal moles of S, Ga, In, As, Ge, R¹, R², M¹, M², Ln and Hal). Forexample, the sulfur based glass compositions according to the inventioncan contain 0-5 mol % In and 20-25 mol % Ga, or 0-12 mol % In and 0-12mol % Ga, or 20-25 mol % In and 0-5 mol % Ga.

In the sulfur based compositions the amount of arsenic is 0-40.00 mol %,based on the total moles (e.g., based on total moles of S, Ga, As, Ge,R¹, R², M¹, M², Ln and Hal). According to another aspect, the sulfurbased glass compositions according to the invention contain, forexample, 0-10.00 mol % As, or 10.00-25.00 mol % As, or 25.00-35.00 mol %As, or 35.00-40.00 mol % As.

According to another aspect, in the sulfur based glass compositionsaccording to the invention a portion of the arsenic can be replaced byantimony, particularly in situations were a lower amount of visibletransmission is acceptable. The presence of Sb tends to reduce visibletransmission. However, the combined total amount of arsenic and antimonyis still preferably 0-40.00 mol %, based on the total moles (e.g., basedon total moles of S, Ga, As, Sb, Ge, R¹, R², M¹, M², Ln and Hal). Forexample, the sulfur based glass compositions according to the inventioncan contain 0-10 mol % Sb and 0-30 mol % As, or 0-20mol % Sb and 0-20mol % As, or 0-30 mol % Sb and 0-10 mol % As.

In the sulfur based compositions the amount of germanium is 0-35.00 mol%, based on the total moles (e.g., based on total moles of S, Ga, As,Ge, R¹, R², M¹, M², Ln and Hal). According to another aspect, the sulfurbased glass compositions according to the invention contain 0-25.00 mol% Ge, for example, 5.00-25.00 mol % Ge, or 10.00-20.00 mol % Ge, or20.00-25.00 mol % Ge.

In the sulfur based compositions the total combined amount of Ga, As,and Ge is 10.00-42.00 mol %, based on the total moles (e.g., based ontotal moles of S, Ga, As, Ge, R¹, R², M¹, M², Ln and Hal). According toanother aspect, the sulfur based glass compositions according to theinvention contain, for example, a total combined amount of Ga, As, andGe of 20.00-40.00 mol %, or 25.00-40.00 mol %, or 30.00-40.00 mol %.

In the sulfur based compositions the amount of Hal is 0-13.5 mol %,based on the total moles (e.g., based on total moles of S, Ga, As, Ge,R¹, R², M¹, M², Ln and Hal). According to another aspect, the sulfurbased glass compositions according to the invention contain 0-10.00 mol% Hal, for example, 1.00-10.00 mol % Hal, or 2.00-9.00 mol % Hal, or3.00-5.00 mol % Hal.

The selection of halide compounds can affect the critical cooling rateof the glass composition. In general, the halides M²Hal₂(M²═Zn, Cd, orPb) and R²Hal(R²═Ag or Cu) produce glass at lower cooling rates and are,therefore, preferred, while glass made with the halides R¹Hal(R¹═Li, Na,K, Rb, or Cs) and M¹Hal₂(M¹═Mg, Ca, Sr, or Ba) tend to requires morerapid cooling. At a given cooling rate, a higher total halogen contentmay be achieved using M²Hal₂ and R²Hal halides, as compared to R¹Hal andM¹Hal₂.

The addition of chlorine is most efficacious in modifying the visibletransmission and thereby the short wavelength dispersion, which areliked though the Kramers-Kronig relation. The addition of Br has asomewhat larger effect than Cl on increasing thermal expansion andthereby reducing dn/dT which is linked through the Lorenz-Lorentzrelation. Br also has a slightly impact on increasing IR transmissionbut a lower impact on increasing visible/NIR transmission relative toCl. The identity of the alkali elements is also impacts thermalexpansion. Larger alkali ions (Cs) will generally tend to increasethermal expansion compared to smaller ions (Li). On the other hand, theidentity of the alkali element will have very little effect on thetransmission or dispersion.

With regards to the selenium based compositions, the amount of seleniumis 30.00-68.00 mol %, based on the total moles (e.g., based on totalmoles of Se, Ga, Ge, R¹, R², M¹, R², M¹, M², Ln, and Hal¹ or the totalmoles of Se, Ga, In, Ge, R¹, R², M¹, M², Ln, and Hal¹). According toanother aspect, the selenium based glass compositions according to theinvention contain 30.00-65.00 mol % of selenium, for example,30.00-60.00 mol % of selenium, or 30.00-55.00 mol % selenium, or30.00-40.00 mol % selenium, or 40.00-55.00 mol % selenium.

In the selenium based compositions the amount of germanium is 0-25.00mol %, based on the total moles (e.g., based on total moles of Se, Ga,Ge, R¹, R², M¹, M², Ln, and Hal). According to another aspect, theselenium based glass compositions according to the invention contain15-25.00 mol % Ge, for example, 15.00-20.00 mol % Ge, or 20.00-25.00 mol% Ge, or 19.00-23.00 mol % Ge. It should be noted that the presence ofgermanium in the selenium based compositions is preferred as it tends toprevent phase separation. If germanium is not present, then it isdesirable to use high amounts, e.g., of chlorides/bromides to preventphase separation.

Also, in the selenium based compositions the amount of gallium is5-30.00 mol %, based on the total moles (e.g., based on total moles ofSe, Ga, Ge, R¹, R², M¹, M², Ln, and Hal). According to another aspect,the sulfur based glass compositions according to the invention contains5-22.00 mol % Ga for example, 5-20.00 mol % Ga, 5.00-15.00 mol % Ga, or5.00-10.00 mol % Ga, or 6 mol %, 7 mol %, 8 mol %, or 9 mol %.

According to another aspect, in the selenium based glass compositionsaccording to the invention a portion of the gallium can be replaced byindium, particularly in situations were a lower amount of visibletransmission is acceptable. The presence of In tends to reduce visibletransmission. However, the combined total amount of Gallium and Indiumis still preferably 5-30.00 mol %, based on the total moles of Se, Ga,In, Ge, R¹, R², M¹, M², Ln, and Hal¹. For example, the sulfur basedglass compositions according to the invention can contain 0-10 mol % Inand 20-30 mol % Ga, or 5-15 mol % In and 5-15 mol % Ga, or 20-30 mol %In and 0-10 mol % Ga.

In the selenium based compositions the total combined amount of Ga andGe is preferably 20.00-40.00 mol %, based on the total moles of based onthe total moles of Se, Ga, Ge, R¹, R², M¹, M², Ln, and Hal¹. Accordingto another aspect, the sulfur based glass compositions according to theinvention contain, for example, a total combined amount of Ga, As, andGe of 21.00-40.00 mol %, or 25.00-35.00 mol %, or 25.00-30.00 mol %.

In the selenium based compositions the amount of Hal¹ is 5-25 mol %,based on the total moles of Se, Ga, In, Ge, R¹ ,R² , M¹, M², Ln, andHal¹. According to another aspect, the sulfur based glass compositionsaccording to the invention contain 5-15.00 mol % Hal¹, for example,5.00-10.00 mol % Hal¹, or 6.00-9.00 mol % Hal¹, or 7.00-9.00 mol % Hal¹.

As noted above, the selection of halide compounds can affect the coolingrate of the glass composition. In general, the halides M²Hal₂(M²═Zn, Cd,Hg, or Pb) and R²Hal(R²═Ag or Cu) produce glass at lower cooling ratesand are, therefore, preferred, while glass made with the halidesR¹Hal(R¹═Li, Na, K, Rb, or Cs) and M¹Hal₂(M¹═Mg, Ca, Sr, or Ba) tend torequires more rapid cooling. At a given cooling rate, a higher totalhalogen content may be achieved using M²Hal₂ and R²Hal halides, ascompared to R¹Hal and M¹Hal₂.

As mentioned above, the addition of chlorine is most efficacious inmodifying the visible transmission and thereby the short wavelengthdispersion, which are liked though the Kramers-Kronig relation. Theaddition of Br has a somewhat larger effect on increasing thermalexpansion and thereby do/dT which are linked through the Lorenz-Lorentzrelation. Br also has a slightly higher impact on increasingtransmission but its impact on visible transmission is weaker ascompared to Cl. The identity of the alkali elements is also impactsthermal expansion. Larger alkali ions (Cs) will generally tend toincrease thermal expansion compared to smaller ions (Li). On the otherhand, the identity of the alkali element will have very little effect onthe transmission or dispersion. However, Cs is preferred over Na or Kwhen large amount of Hal are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present inventionwill be more fully appreciated as the same becomes better understoodwhen considered in conjunction with the accompanying drawings, in whichlike reference characters designate the same or similar parts throughoutthe several views, and wherein:

FIG. 1 illustrates a doublet lens system containing a corrective lens inaccordance with the invention; and

FIG. 2 illustrates a triplet lens system containing a corrective lensmade from two glasses in accordance with the invention.

As described above, optical materials for IR wavelengths suffer fromthermally-induced changes in focal length in lenses due to thermalexpansion and dn/dT. In an achromatic doublet lens, the dispersionscombine to provide equal powers at 2 wavelengths. To athermalize a lens(i.e., to reduce thermal effects), the coefficient of thermal expansion(CTE) and dn/dT need to be balanced. Therefore, using the followingequations:

${{\alpha \left( {C\; T\; E} \right)} = {\frac{I}{L}\frac{DL}{({DT})}}},{\beta = \frac{n}{t}},{and}$$\mspace{14mu} {{\delta = {\frac{\beta}{n - 1} - {\alpha \left( {{thermal}\mspace{14mu} {change}\mspace{14mu} {in}\mspace{14mu} {focal}\mspace{14mu} {power}} \right)}}},}$

one can estimate the requirements for achieving an athermal andachromatic system.

For a doublet lens, the power, K, is equal to the powers of theindividual lens, i.e., K₁+K₂=K (doublet). For an achromatic lens,K₁/V₁+K₂/V₂=0 (i.e., K₂=K₁V2/V₁) V represents the Abbe No. Forathermalization, K₁δ₁+K₂δ₂=Kα_(h), where α_(h) is the thermal expansioncoefficient of the housing material (i.e., the housing holding thelens). Combining the equations results in δ₂=[V₁(α_(h)−δ₁)/V₂]+α_(h).Thus, the corrective lens of the doublet preferably satisfies thiscriterion.

FIG. 1 illustrates a doublet lens wherein an infrared lens 1 is pairedwith a corrective lens 2 made a chalcogenide glass composition accordingto the invention. The IR lens 1 and corrective lens 2 are preferablyfused together, although they can also be separated by a small airspace. Lens 1 can be made from any of the commonly used material for IRlenses, for example, ZnSe, ZnS, Ge, GaAs, BaF₂, and chalcogenideglasses, preferably ZnSe or ZnS. Preferably, a ZnSe IR lens is pairedwith a corrective lens made from a selenium based glass compositionaccording to the invention, as these glasses will have similartransmission properties. For similar reasons, a ZnS IR lens ispreferably paired with a corrective lens made form a sulfur based glasscomposition according to the invention.

As shown in FIG. 1, light passing through the IR lens 1 is subjected todispersion due to the variance in refractive index, which causes thefocal length to be shorter at shorter wavelengths. This light thenpasses through corrective lens 2 which corrects the light transmissionby preferentially increasing the focal length relative to that createdby the first lens at shorter wavelengths, thereby counteracting theeffects of the first lens.

FIG. 2 illustrates another embodiment according to the invention whereina pair of lenses may be added to the system in order to leave the focallength at a single wavelength unaffected but to change either thedispersive or thermal behavior of the system in order to counteract theeffects of the main focusing element. This is most efficacious incorrecting problems in existing systems. For instance, thermal defocusin Germanium-based optical systems may be corrected by inserting a lenspair with an infinite focal length at room temperature, but whichbecomes negative at elevated temperatures or positive at decreasedtemperatures, thereby correcting the errors introduced by the germaniumelement.

Thus, an existing lens may be corrected using two lenses (one positiveand one negative) which give a total power of 0 (afocal) at the centerwavelength, but which have different V and δ to correct deficiencies ofthe primary lens without change overall focal length. Thus, the powersof the two corrective lenses are to cancel each other out, i.e.,K₁+K₂+K₃=K₁ when K₂=−K₃ (K₁ is the power of the existing lens and K₂ andK₃ are the powers of the doublet lens). Going through the process ofachromatizing and althermalizing using the equations described above,the 2 glasses of the doublet lens preferably satisfy the following (witha preference for small K₂): [V₂V₃/V₁(V₃−V₂)=(α_(h)−δ₁)/(δ₂.δ₃).

Thus, in FIG. 2 an infrared lens 1 is used in combination with a doubletlens containing lens elements 2 and lens 3, one having a negative powerand the other having a positive power. The negative power lens elementshould have higher dispersion (smaller V) and higher δ than positivelens element. Lens 2 and lens 3 are preferably fused. Lens 1 can be madefrom any of the commonly used material for IR lenses, for example, ZnSe,ZnS, Ge, GaAs, BaF₂, and known chalcogenide glasses, preferably ZnSe orZnS. At least one of lens 2 and lens is made from a chalcogenide glasscomposition according to the invention. The other lens can be made froma chalcogenide glass composition according to the invention or from anyof the commonly used material for IR lenses, such as ZnSe, ZnS, Ge,GaAs, BaF₂, and known chalcogenide glasses. For example, lens 1 can beof ZnS, lens 2 can be from a chalcogenide glass composition according tothe invention, and lens 3 can be made of ZnSe.

EXAMPLES

The glasses of this invention can be fully conventionally prepared bymixing the appropriate amounts of each constituent to form a batchcomposition which is then charged into a fused silica ampoule and meltedby radiative heating, e.g., from 600° C. to as much as 1050° C.,depending on the chosen composition typically 2 to 4 hours, againdepending on composition and melt viscosity while rocking the melt inorder to cause agitation and increase homogeneity. The glass within itsampoule is then typically removed from the furnace and allowed to coolby convection in room temperature air to a temperature near its glasstransition temperature. The ampoule and glass sample are then placedinto a heated oven at the glass transition temperature plus about 20° C.for about 2 hours followed by cooling at about 30° C./hour. Theseprocedures are followed in the examples below.

As noted above, the examples of this application are melted in a fusedsilica ampoule. It is well known that chalcogenide compounds,particularly those of S with Ge or Ga possess high vapor pressures nearthe melt temperature. The pressure evolved during melting may exceed theburst pressure of the silica vessel, leading to rupture of the ampoule.Also, the thermal expansion of these glasses is relatively largecompared to that of the ampoule. Under the conditions of wetting of theglass to the interior of the ampoule, the stress induced duringquenching may cause a rupture ampoule and/or glass ingot within. Thetemperatures and heating rates during the melting and quenchingoperations must therefore be chosen judiciously in order to preventrupture, depending on the design of the ampoule and dimensions andcomposition of the glass ingot. The need to control these factors whilestill providing sufficiently high melting temperatures and cooling rateswhile quenching combine to limit the dimensions of the ampoule and glasssample which may be prepared.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

Tables 1A, 1B, 1C, 1D, 1E, and 1F list examples of the glass compositionaccording to the invention. Tables 1A-1D list examples of the sulfurbased glass composition and Tables lE and 1F lists examples of theselenium based glass composition.

TABLE 1A Examples of Sulfur Based Glass Compositions (mol %) Accordingto the Invention Component Content (mol %) 1 2 3 4 5 6 7 S 60 60 60 6558 65 70 Ge 5 10 10 20 25 23 Ga As 40 35 30 25 12 10 7 Total 100 100 100100 100 100 100

TABLE 1B Further Examples of Sulfur Based Glass Compositions (mol %)According to the Invention Component Content Examples (mol %) 8 9 10 11S 70 70 75 70 Ge 25 20 20 23 Ga 5 10 5 7 As Total 100 100 100 100

TABLE 1C Examples of Selenium Based Glass Compositions (mol %) Accordingto the Invention Component Content Examples (mol %) 12 13 14 15 16 17 1819 Se 37 37 52.5 54.2 55.6 52.5 54.2 55.6 Ga 21 21 9.5 8.3 7.4 9.5 8.37.4 Ge 19 20.9 22.2 19 20.9 22.2 Br 21 9.5 8.3 7.4 Cl 21 9.5 8.3 7.4 Cs,Na, 21 21 9.5 8.3 7.4 9.5 8.3 7.4 K, Ag (Cs) (Cs) (Na) (Na) (Na) (Na)(Na) (Na) Total 100 100 100 100 100 100 100 100

TABLE 1D Examples of Selenium Based Glass Compositions (mol %) Accordingto the Invention Component Content Examples (mol %) 20 21 22 23 24 25 2627 Se 37 37 52.5 54.2 55.6 52.5 54.2 55.6 Ga 21 21 9.5 8.3 7.4 9.5 8.37.4 Ge 19 20.9 22.2 19 20.9 22.2 Br 21 9.5 8.3 7.4 Cl 21 9.5 8.3 7.4 Cs,Na, 21 21 9.5 8.3 7.4 9.5 8.3 7.4 K, Ag (Ag) (Ag) (K) (K) (K) (K) (K)(K) Total 100 100 100 100 100 100 100 100

TABLE 1E Further Examples of Selenium Based Glass Compositions (mol %)According to the Invention Component Content Examples (mol %) 28 29 3031 32 33 Se 55 65.5 57.7 55 65.5 57.7 Ga 10 8.7 7.7 10 8.7 7.7 Ge 2021.8 23 20 21.8 Br 10 8.7 7.7 Cl 10 8.7 7.7 Zn 5 4.3 3.9 5 4.3 3.9 Total100 100 100 100 100 100

TABLE 1F Further Examples of Selenium Based Glass Compositions (mol %)According to the Invention Component Content Examples (mol %) 34 35 3637 38 39 Se 55 65.5 57.7 55 65.5 57.7 Ga 10 8.7 7.7 10 8.7 7.7 Ge 2021.8 23 20 21.8 Br 10 8.7 7.7 Cl 10 8.7 7.7 Pb 5 4.3 3.9 5 4.3 3.9 Total100 100 100 100 100 100

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The entire disclosure[s] of all applications, patents and publications,cited herein, are incorporated by reference herein.

1. A chalcogenide glass composition comprising (based on mol % of totalmoles): Component Mole % S 58.00-90.00    Ga 0-25.00 As 0-40.0  Ge0-35.00 R¹ 0-7.25  (added in the form of R¹Hal) R² 0-13.5  (added in theform of R²Hal) M¹ 0-5    (added in the form of M¹Hal₂) M² 0-7.25  (addedin the form of M²Hal₂) Ln 0-4    (added in the form of LnHal₃) Sum ofGa, As, and Ge 10.00-42.00    Sum of R¹, R², M¹, M², and Ln 0-16.00 Sumof Hal 0-16.00

wherein Hal=fluoride, chloride, bromide, and/or iodide, R¹=Li, Na, K,Rb, and/or Cs, R²=Ag and/or Cu, M¹=Mg, Ca, Sr, and/or Ba, M²=Zn, Cd, Hg,and/or Pb, and Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Ty, Lu, Y, and Sc; wherein in each of (a) and (b) a portion of thegallium can be replaced by indium, and wherein a portion of the arseniccan be replaced by antimony.
 2. A chalcogenide glass compositionaccording to claim 1, wherein said composition comprises (based on mol%): Component Mole % S 58.00-90.00    Ga 0-25.00 As 0-40.0  Ge 0-35.00R¹ 0-7.25  (added in the form of R¹Hal) R² 0-13.5  (added in the form ofR²Hal) M¹ 0-5    (added in the form of M¹Hal₂) M² 0-7.25  (added in theform of M²Hal₂) Ln 0-4    (added in the form of LnHal₃) Sum of Ga, As,and Ge 10.00-42.00    Sum of R¹, R², M¹, M², and Ln 0-16.00 Sum of Hal 0-16.00.


3. (canceled)
 4. A chalcogenide glass composition according to claim 2,wherein said composition comprises (based on mol %): Component Mole % S65.00-75.00 Ga    0-10.00 As    0-35.00 Ge  3.00-30.00 R¹ 0-5 (added inthe form of R¹Hal) R²  0-10 (added in the form of R²Hal) M¹ 0-3 (addedin the form of M¹Hal₂) M² 0-5 (added in the form of M²Hal₂) Ln 0-3(added in the form of LnHal₃) Sum of Ga, As, and Ge 30.00-40.00 Sum ofR¹, R², M¹, M²  0-10 and Ln Sum of Hal   0-10.


5. (canceled)
 6. A chalcogenide glass composition according to claim 2,wherein said composition contains 58.00-75.00 mol % of sulfur.
 7. Achalcogenide glass composition according to claim 2, wherein saidcomposition contains 0-20.00 mol % Ga.
 8. A chalcogenide glasscomposition according to claim 2, wherein a portion of the gallium isreplaced by indium.
 9. A chalcogenide glass composition according toclaim 2, wherein said composition contains 35.00-40.00 mol % As.
 10. Achalcogenide glass composition according to claim 2, wherein a portionof the arsenic is replaced by antimony.
 11. A chalcogenide glasscomposition according to any one of claims 2 and 6 to 10, wherein saidcomposition contains 0-25.00 mol % Ge.
 12. A chalcogenide glasscomposition according to claim 2, wherein said composition contains0-10.00 mol % Hal.
 13. (canceled)
 14. (canceled)
 15. (canceled) 16.(canceled)
 17. (canceled)
 18. In a night vision device comprising aninfrared optical element, an image enhancer or intensifier, and aphosphor or fluorescent display, the improvement wherein said infraredoptical element comprises a lens made of from a chalcogenide glasscomposition according to claim
 1. 19. In an infrared or thermal imagingsystem comprising an infrared optical element, a plurality of thermalsensors for detecting the infrared light and converting it intoelectrical signals, and a signal-processing unit for converting theelectrical signals into a visual image, the improvement wherein saidinfrared optical element comprises a lens made of from a chalcogenideglass composition according to claim
 1. 20. A doublet lens comprising aninfrared lens paired with a corrective lens wherein said infrared lensis made of ZnSe, ZnS, Ge, GaAs, BaF₂, or chalcogenide glass, and saidcorrective lens made from a chalcogenide glass composition according toclaim
 1. 21. An infrared lens system comprising a first infrared lensand a focal corrector doublet lens comprising a pair of correctivelenses, wherein said first infrared lens is made of ZnSe, ZnS, Ge, GaAs,BaF₂, or chalcogenide glass, one of said pair of corrective lenses has apositive power and the other has a negative power, and at least one ofsaid pair of corrective lens is made from a chalcogenide glasscomposition according to claim 1.